Because this is what you have to believe if you think the gap in singleness in the Pew survey is caused by women 'sharing the same guy'.

These are the figures broken down by relationship type for the Pew survey earlier in the same year as the famous one, recruiting participants from the same panel so probably a good amount of overlap:

Pew Feb 2022 Married Cohabiting Non-cohabiting Single
18-29 Men 16% 9% 18% 57%
18-29 Women 20% 26% 22% 32%

So the gap exists mostly in the cohabiting category, with 4% in either the married or non-cohabiting categories.

The same thing was also seen in the recent 2022 American Perspectives survey (and the earlier one in 2020), which also showed a much lower gap in singleness of 12%:

https://www.americansurveycenter.org/research/from-swiping-to-sexting-the-enduring-gender-divide-in-american-dating-and-relationships/

3% of the gap was in non-cohabitation (though 3% of men also selected 'other', while 5% was in marriage and 5% was in cohabitation.

This means that the notion that there's a significant trend of men 'spinning plates' which explains the singleness gap is highly questionable, as the only gap that could 'realistically' operate within is non-cohabition.

Since polygamy is illegal in the US last I checked any kind of harem based on that gap is out of the question too.

So what we're left with outside of that at most minimal phenomenon which could exist in the non-cohabiting category (even there though the 2017-19 NSFG showed 1% of both 18-29 men and women were non-cohabiting with more than one person) is more young women than men cohabiting with their partners. This gap could either mean they are cohabiting with men outside the age bracket, with other women, or with a smaller number of men in the same age bracket. Even assuming the 29% gap is real (while other sources such as again the APS and also the GSS show much smaller gaps), it just doesn't seem likely that the 'sharing the same guy' explanation is plausible unless you think there's a widespread trend of polyamory, and not just that but the cases in which there is a single man living with multiple women far outnumbers the other cases, which based on the only data I've seen on this isn't even the case. Why has nobody noticed this massive polyamory phenomenon if it's real? Are you aware of any cases at all of multiple women shacking up with the same guy? I don't think I've ever seen this.